Since an amplified image sensor such as a CMOS image sensor has advantages such as a low-voltage operation, a low power consumption, ease of being integrally formed with peripheral circuits, and a low cost compared to a charge transfer image sensor such as a charge coupled device (CCD) image sensor, the amplified image sensor is mounted in various electronic devices such as a digital camera, a surveillance camera, and a mobile phone camera.
A solid-state image sensor includes a pixel array in which pixel circuits generating charges via photoelectric conversion are arrayed. Here, a typical pixel array will be described with reference to the accompanying drawings. FIG. 30 is a schematic diagram illustrating a typical pixel array.
As illustrated in FIG. 30, pixel circuits are arrayed in a matrix form in a typical pixel array 100. The typical pixel array 100 includes optically black pixel circuits (hereinafter, referred to as “OB pixel circuits”) POB which are shielded from light in addition to pixel circuits of effective pixels (hereinafter, referred to as “effective pixel circuits”) PN on which light is incident (which are exposed to light). Since a signal or data obtained from the OB pixel circuit POB contains only unnecessary components such as dark current or noise, the signal or data obtained from the OB pixel circuit POB is used when performing an offset correct process by which unnecessary components such as dark current or noise are removed from a signal or data obtained from the effective pixel circuit PN.
In a charge transfer image sensor, charges generated by each pixel circuit are sequentially transferred to pixel circuits which are adjacent to each other in a predetermined transfer direction (for example, pixel circuits in the same column which are adjacent to each other in the pixel array 100 in a vertical direction in FIG. 30), and then signals are sequentially acquired in correspondence with the charges. In contrast, in an amplified image sensor, it is possible to selectively acquire a signal corresponding to charges generated in any pixel circuit. In a typical amplified image sensor, a signal line is shared with multiple pixel circuits (for example, pixel circuits in the same row which are arrayed in the pixel array 100 in a lateral direction in FIG. 30) from the viewpoint of simplifying the configuration and control of the amplified image sensor, and speeding up the operation, and the operation is controlled in the unit of the multiple pixel circuits (hereinafter, referred to as a “control group”).
In the charge transfer image sensor, if strong light is incident on a portion of pixel circuits, charges superabundantly generated by the pixel circuits overflow along a transfer direction, and therefore the amount of charges in pixel circuits which are arrayed in the transfer direction increases as a whole. Accordingly, white stripes (smear) occur in eventually obtained image data along the transfer direction. In contrast, in the amplified image sensor, in a case where strong light is incident on a portion of pixel circuits, even if the overflow of charges into peripheral pixel circuits (blooming) may occur, charges do not overflow along a specific direction. Therefore, in principle, smear does not occur.
However, in the amplified image sensor, if strong light is incident on a portion of pixel circuits, other pixel circuits belonging to the same control group, where the pixel circuits in the portion belong, are affected, and therefore a defect (hereinafter, referred to as “pseudo-smear”) similar to smear may occur in eventually obtained image data.
A cause of the occurrence of pseudo-smear will be described with reference to the accompanying drawings. FIG. 31 is a circuit diagram of a pixel circuit illustrating a cause of pseudo-smear occurring in a solid-state imaging device in the related art. FIG. 32 is a timing chart illustrating the operation of the pixel circuit illustrated in FIG. 31, on which weak light is incident. FIG. 33 is a timing chart illustrating the operation of the pixel circuit illustrated in FIG. 31, on which strong light is incident.
As illustrated in FIG. 31, the effective pixel circuit PN and the OB pixel circuit POB have a similar configuration. Each of the effective pixel circuit PN and the OB pixel circuit POB includes a photodiode PD that generates charges via photoelectric conversion; a floating diffusion area FD that retains charges transferred from the photodiode PD; a transfer gate 101 through which charges are transferred from the photodiode PD to the floating diffusion area FD; an output transistor 102 that outputs a signal (voltage) corresponding to the amount of charges retained by the floating diffusion area FD; and a reset transistor 103 that discharges charges in the photodiode PD and the floating diffusion area FD to the outside of the effective pixel circuit PN and the OB pixel circuit POB. In the effective pixel circuit PN and the OB pixel circuit POB illustrated in FIG. 31, charges accumulated by the photodiode PD and retained by the floating diffusion area FD are electrons, and each transistor is an N-channel field effect transistor (FET).
An anode of the photodiode PD is grounded. The transfer gate 101 is connected to a transfer control line TX, and is a gate of a transistor, the drain of which is the floating diffusion area FD, and the source of which is a cathode of the photodiode PD. The output transistor 102 has a gate connected to the floating diffusion area FD, a drain connected to a common power supply line VD, and a source connected to an output signal line VS. The reset transistor 103 has a gate connected to a reset control line RST, a drain connected to a reset power supply line VR, and a source connected to the floating diffusion area FD.
The effective pixel circuit PN and the OB pixel circuit POB illustrated in FIG. 31 belong to the same control group. Accordingly, the transfer control line TX, the reset control line RST, and the reset power supply line VR are common to the pixel circuits PN and POB. The common power supply line VD is common to all the pixel circuits PN and POB in the pixel array 100.
As illustrated in FIG. 31, the effective pixel circuit PN includes a parasitic capacitor CP1 between the output signal line VS and the transfer control line TX. The OB pixel circuit POB includes a parasitic capacitor CP2 between the floating diffusion area FD and the transfer control line TX. In FIG. 31, only the parasitic capacitors CP1 and CP2 specifically related to the aforementioned problem are illustrated, and the illustration of other parasitic capacitors is omitted.
The effective pixel circuit PN and the OB pixel circuit POB perform correlated double sampling (CDS) by which a difference between the voltage of the output signal line VS when the floating diffusion area FD is reset and the voltage of the output signal line VS when charges are transferred from the photodiode PD to the floating diffusion area FD. Effective pixel data for image data and OB pixel data are obtained by performing analog-to-digital (A/D) conversion on the differences obtained from the effective pixel circuit PN and the OB pixel circuit POB.
Specifically, as illustrated in FIGS. 32 and 33, first, during a period Tin, the reset control line RST goes to a high voltage H, and therefore the reset transistor 103 enters an ON mode (state in which a gate-to-source voltage is higher than a threshold voltage, and this is the same in the following description), and charges retained by the floating diffusion area FD are discharged to the outside of the pixel circuits PN and POB via the reset power supply line VR at the high voltage H. Subsequently, during a period T102, the reset control line RST goes to a low voltage L, and therefore the reset transistor 103 enters an OFF mode (contrary to an ON mode, a state in which the gate-to-source voltage is lower than or equal to the threshold voltage, and which may include a case where leak current or the like flows, and this is the same in the following description). At the end of the period T102, a voltage VrN of the output signal line VS in a state where charges are not retained in the floating diffusion area FD of the effective pixel circuit PN and a voltage VrOB of the output signal line VS in a state where charges are not retained in the floating diffusion area FD of the OB pixel circuit POB are sampled. Subsequently, during a period T103, the transfer control line TX goes to the high voltage H, and therefore a transistor including the transfer gate 101 as a gate enters an ON mode, and charges in the photodiode PD are transferred to the floating diffusion area FD. Subsequently, during a period T104, the transfer control line TX goes to the low voltage L, and therefore the transistor including the transfer gate 101 as a gate enters an OFF mode. At the end of the period T104, a voltage VsN of the output signal line VS which corresponds to charges retained in the floating diffusion area FD of the effective pixel circuit PN and a voltage VsOB of the output signal line VS which corresponds to charges retained in the floating diffusion area FD of the OB pixel circuit POB are sampled. In this case, a correlated double sampled difference obtained from the effective pixel circuit PN is VrN−VsN, and a correlated double sampled difference obtained from the OB pixel circuit POB is VrOB−VsOB. Effective pixel data for image data and OB pixel data are obtained by performing A/D conversion on the differences.
In FIGS. 32 and 33, the voltage of the output signal line VS of the OB pixel circuit POB fluctuates during the period T104. The reason for this is that a fluctuation in the voltage of the output signal line VS of the effective pixel circuit PN is transmitted to the floating diffusion area FD of the OB pixel circuit POB via the parasitic capacitor CP1, the transfer control line TX, and the parasitic capacitor CP2. In FIGS. 32 and 33, a fluctuation in the voltage of the output signal line VS is illustrated in an emphasized manner, and the illustration of noise superimposed on the transfer control line TX or the reset control line RST is omitted.
As illustrated in FIG. 32, if light incident on the effective pixel circuit PN is weak, and a fluctuation in the voltage of the output signal line VS is small, a fluctuation in the voltage of the output signal line VS of the OB pixel circuit POB also is small. Accordingly, the correlated double sampled difference VrOB−VsOB is close to zero, and obtained OB pixel data is close to the minimum value. Since OB pixel data is data obtained from the OB pixel circuit POB which is shielded from light as described above, in an ideal mode in which there is no dark current, noise, or the like, the OB pixel data is data that has to be the minimum value.
In contrast, as illustrated in FIG. 33, if light incident on the effective pixel circuit PN is strong, and a fluctuation in the voltage of the output signal line VS of the effective pixel circuit PN is large, a fluctuation in the voltage of the output signal line VS of the OB pixel circuit POB also is large. Accordingly, the correlated double sampled difference VrOB−VsOB increases, and obtained OB pixel data also increases.
Although FIG. 33 illustrates mainly the OB pixel circuit POB, other effective pixel circuits PN, on which strong light is not incident and which belong to the same control group as that of the effective pixel circuit PN on which strong light is incident, receive the same effect as that on the OB pixel circuit POB. That is, effective pixel data obtained from the other effective pixel circuits PN also increases from original unaffected values. Accordingly, there is an increase as a whole in effective pixel data and OB pixel data obtained from the pixel circuits PN and POB belonging to the same control group as that of the effective pixel circuit PN on which strong light is incident. An increase in data of the effective pixels and the OB pixels along the control group appears as white (bright) pseudo-smear in image data.
In a case where pseudo-smear has occurred as described above, each of effective pixel data and OB pixel data increases. Therefore, it is possible to reduce or eliminate pseudo-smear by performing an offset correction process on the effective pixel data based on the OB pixel data. The offset correction process will be described with reference to the accompanying drawings. FIGS. 34 and 35 are schematic views illustrating an offset correction process in a case where pseudo-smear has occurred. FIGS. 34(a) and 35(a) illustrate image data on which an offset correction process is not performed. FIGS. 34(b) and 35(b) illustrate image data on which an offset correction process is performed.
Since strong light is incident on the effective pixel circuits PN at the center of the pixel array 100, a high-luminance area occurs at a position (center) corresponding to the effective pixel circuits PN in the image data illustrated in FIG. 34(a). As a result, pseudo-smear has occurred in a lateral direction (row direction) in FIG. 34(a). If an offset correction process (for example, a correction process of subtracting OB pixel data from effective pixel data) is performed on effective pixel data containing pseudo-smear such that an OB pixel containing the pseudo-smear illustrated in FIG. 34(a) become a black pixel, as illustrated in FIG. 34(b), it is possible to reduce or eliminate the pseudo-smear via cancellation.
However, in a case where an offset correction process is performed, it may not be possible to reduce or eliminate pseudo-smear, and image data may be adversely affected. The image data illustrated in FIG. 35(a) contains a high-luminance area at the same position as that in the image data illustrated in FIG. 34(a), and pseudo-smear appears stronger than that in the image data illustrated in FIG. 34(a) (that is, the amount of increase in data due to pseudo-smear is large). In this case, if an offset correction process is performed, the effective pixel is over-corrected based on the OB image data which is excessively large. As a result, pseudo-smear which is more black (darker) than the surrounding area may remain, which is a problem (a black pseudo-smear will be described later with reference to FIG. 3).
In order to reduce the occurrence of pseudo-smear, PTL 1 proposes a solid-state image sensor that performs correction by promptly absorbing a fluctuation in the output of an OB pixel circuit via a decrease in OB clamp time constant in a case where an AGC gain is greater than a predetermined value, and a high-luminance object detecting circuit detects a high-luminance object. PTL 2 proposes a solid-state image sensor that sets a voltage based on a pixel reset potential and, performs correction in correspondence with the threshold voltage of an element of control means so as to clip a lower limit of the potential of a vertical signal line at a clip potential slightly lower than a saturation potential.
PTL 3 proposes a solid-state image sensor that not only sequentially performs the operation of an electronic shutter (start of the accumulation of charges), the accumulation of charges, and the reading of charges (transfer of charges accumulated in a photodiode to a floating diffusion area), but also clears charges accumulated by a photodiode after the reading of charges and during a stand-by period before the operation of the electronic shutter so as to prevent charges, which are accumulated by the photodiode, from overflowing into adjacent pixel circuits and the like (blooming).